Bulletproof glass
Bulletproof glass is a type of strong but optically transparent material that is particularly resistant to being penetrated when struck by bullets, but is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic, so it can flex instead of shatter. The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear view (not distorted) through the glass. Bulletproof glass varies in thickness from three-quarter inch to three inches.

Another construction which is maturing fast is the use of security laminates as a film on the inner surface of ordinary glass. This when bonded with the application of a pressure sensitive adhesive and cured fully, also provides a protection similar to the multi-layered bullet-resistant glass. The optical clarity is much better, tint-free, thickness and weight reduced as much as 50-70% and the process can be done as a retrofit on existing windows.


Bullet-resistant glass is usually constructed using polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

, thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

, and layers of laminated glass
Laminated glass
Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral , between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high...

. The aim is to make a material with the appearance and clarity of standard glass but with effective protection from small arms. Polycarbonate designs usually consist of products such as Armormax, Makroclear, Cyrolon, Lexan or Tuffak, which are often sandwiched between layers of regular glass.
The ability of a glass itself to withstand shock is improved by the process of tempering. When treated with heating and cooling or with chemical processes, the glass becomes much stronger. The polycarbonate usually has one of two types of coating to resist abrasion: a soft coating that heals after being scratched (such as elastomeric carbon-based polymers) or a hard coating that prevents scratching (such as silicon-based polymers)

The plastic in laminate designs also provides resistance to impact from physical assault from hammers, axes, clubs, and so forth. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet, and the plastic deforms, (hopefully) absorbing the rest of the energy and preventing penetration. The ability of the polycarbonate layer to stop projectiles with varying energy is directly proportional to its thickness, and bulletproof glass of this design may be up to three inches thick.

Laminated glass layers are built from glass sheets bonded together with polyvinyl butyral, polyurethane or ethylene-vinyl acetate. This design has been in regular use on combat vehicles since World War II; it is typically thick and is usually extremely heavy.
Sample thickness and areal densities for bullet-resistant glass materials
NIJ Threat Stopped Glass Laminate Polycarbonate Acrylic Glass-Clad Polycarbonate
Protection Level (example) Thickness Density Thickness Density Thickness Density Thickness Density
in. lb/sq. ft. in. lb/sq. ft. in. lb/sq. ft. in. lb/sq. ft.
I 9 mm 1.185 15.25 0.75 4.6 1.25 7.7 0.818 8.99
II .357 Magnum 1.4 17.94 1.03 6.4 1.375 8.5 1.075 11.68
III 44 Magnum 1.59 20.94 1.25 7.7 1.288 14.23
IV 30 Caliber 1.338 14.43
V 30 Caliber 1.338 14.43
VIII 7.62 mm 2.374 26.01

Test standards

Bullet-resistant materials are usually tested by using a gun to fire a projectile from a set distance into the material in a set pattern. Levels of protection are based on the ability of the target to stop a specific type of projectile traveling at a specific speed. Experiments suggest that polycarbonate fails at lower velocities with regular shaped projectiles compared to irregular ones (like fragments), so that testing with regular shaped projectiles probably gives a conservative estimate of its resistance. When projectiles do not penetrate, the depth of the dent left by the impact can be measured and related to the projectile’s velocity and thickness of the material. Some researchers have developed mathematical models based on results of this kind of testing to help them design bulletproof glass to resist specific anticipated threats.

Well known standards for categorizing ballistic resistance include the following:

Environmental effects

The properties of bullet-resistant glass can be affected by temperature and by exposure to solvents or UV radiation
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

, usually from sunlight. If the polycarbonate layer is below a glass layer, it has some protection from UV radiation due to the glass and bonding layer. Also, over time the polycarbonate becomes more brittle because it is an amorphous polymer (which is necessary for it to be transparent) that moves toward thermodynamic equilibrium.

Impact of polycarbonate by a projectile at temperatures below -7oC sometimes creates spall
Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure...

, pieces of polycarbonate that are broken off and become projectiles themselves. Experiments have demonstrated that the size of the spall is related to the thickness of the laminate rather than the size of the projectile. The spall starts in surface flaws caused by bending of the inner, polycarbonate layer and the cracks move “backwards” through to the impact surface. It has been suggested that a second inner layer of polycarbonate may effectively resist penetration by the spall.


Advances in bullet-resistant glass have led to the invention of one-way bulletproof glass, such as used in some armored trucks
Armored car (valuables)
A common meaning of armored car is as an armored van or truck, used in transporting valuables, such as large quantities of money . The armored car is a multifunctional vehicle designed to protect and ensure the well being of the transported individuals and/or contents...

. This glass resists incoming small arms fire striking the outside, but will allow those on the other side, such as guards inside the vehicle, to fire out through the glass at the exterior threat.

One-way bulletproof glass is usually made up of two layers, a brittle layer on the outside and a flexible one on the inside. A bullet fired from the outside hits the brittle layer first, shattering an area of it. This absorbs some of the bullet's kinetic energy, and spreads it on a larger area. When the slowed bullet hits the flexible layer, it is stopped. However, when a bullet is fired from the inside, it hits the flexible layer first. The bullet penetrates the flexible layer because its energy is focused on a smaller area; the brittle layer then shatters outward due to the flexing of the inner layer and does not significantly hinder the bullet's progress. One-way bullet-resistant glass is far from being perfected; there is evidence that suggests it can be achieved, but in most cases when shooting from the "safe" side, the intended target would have to be very close for the bullet to cause lethal wounds.

Recent advances

U.S. military researchers are developing a new class of transparent armor incorporating
aluminum oxynitride
Aluminium oxynitride
Aluminium oxynitride or AlON is a transparent polycrystalline ceramic with cubic spinel crystal structure composed of aluminium, oxygen and nitrogen. It is currently marketed under the name ALON by Surmet Corporation. ALON is optically transparent in the near ultra violet, visible and near...

 (Trade name: ALON) as the outside "strike plate" layer. It is much lighter and performs much better than traditional glass/polymer laminates. Aluminum oxynitride "glass" can defeat threats like the .50 caliber armor piercing rounds using material that is not prohibitively heavy. Various types of other materials which closely resemble glass are also being developed.

Spinel ceramics

Certain types of ceramic spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

 (a class of mineral) can also be used for transparent armor due to their properties of increased density and hardness when compared to traditional glass. These new types of synthetic ceramic transparent armors can allow for thinner armor with equivalent stopping power to traditional laminated glass.

See also

  • Transparent Armor Gun Shield
    Transparent Armor Gun Shield
    Built by BAE Systems, the Transparent Armor Gun shield, or TAGS, is a visually transparent protective gun shield for operators of vehicle-mounted machine guns. It borrows on the experience of the Israeli Defense Force in using such armor on a variety of vehicles. The shield is intended to provide...

  • Prince Rupert's Drop
    Prince Rupert's Drop
    Prince Rupert's Drops are a glass curiosity created by dripping hot molten glass into cold water. The glass cools into a tadpole-shaped droplet with a long, thin tail. The water rapidly cools the molten glass on the outside of the drop, while the inner portion of the drop remains significantly...

  • Access control vestibule
    Access control vestibule
    The Access Control Vestibule is a security screening system for the detection of individuals carrying weapons into any facility without authorization, therefore minimizing the possibility of an armed takeover. The ACV is composed of an aluminum frame, separate entrance and exit doors, a metal...

External Links

A Brief History of Glass
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.